1. Technical Field
Embodiments of the present invention relate generally to energy and power management or monitoring and, more particularly, to systems and methods for measuring neutral-to-ground voltages and reducing ground currents caused by the measurements.
2. Background Discussion
Providing relevant energy and power management solutions requires knowledge, and knowledge requires accurate data. Under certain circumstances, however, the act of acquiring data can undermine the validity of the data and may also adversely impact the system being measured.
Poor wiring and grounding practices have been found to be the primary cause of all power quality issues according to well-known Electric Power Research Institute (EPRI) studies conducted in the 1990s. Poorly designed and installed electrical systems not only can cause erratic operation of equipment, they can result in safety or fire hazards.
Wiring and grounding practices are critical to the safe and reliable operation of electrical systems. The National Fire Protection Agency (NFPA) produces a codebook (the NEC or National Electrical Code) that outlines appropriate wiring practices to provide “the practical safeguarding of persons and property from hazards arising from the use of electricity.” This code is very thorough, and is relied upon by many international jurisdictions to reduce the risks associated with electricity, safeguarding equipment and personnel; it is essentially an instruction manual for the safe design and installation of wiring and grounding systems. The IEEE Emerald Book (IEEE Standard 1100-2005, Recommended Practice for Powering and Grounding Electronic Equipment) takes the NEC a step further and provides design and installation guidelines to reduce the impact of poor wiring and grounding practices where they could result in reliability or power quality issues (Note: IEEE Standard 1100-2005 is only a supplement to the NEC; in no way does it supplant the NEC).
Measuring the voltage potential between the grounded (neutral) connection and the ground reference provides valuable information with respect to safety and potential power quality issues. With that in mind, many meter manufacturers are now employing the capability to measure neutral-to-ground (N-G) voltages, even in their low-end devices. Some exemplary benefits of measuring and analyzing N-G voltage data include the following: detecting illegal N-G bonds (violations of the NEC); indicating of the need to consider installing a new separately derived source; distinguishing the relative location of a device (with respect to other devices) within an electrical subsystem; identifying potential impedance issues with the grounded (neutral) conductor; and verifying the continuity of grounded (neutral) and grounding (earth) conductors.
The 2011 NEC dedicates an entire section of the code (Article 250) to provide detailed instructions on configuring grounded electrical systems. For ease of reference and clarity of exposition, so that the concepts and embodiments described herein may be more clearly understood, the following sets forth the meaning of various terms as may be used herein, which meaning is also intended to be in accordance with the definition and use of these terms in the 2011 NEC Handbook, from which the following is directly based.
Bonded (Bonding)—Connected to establish electrical continuity and conductivity.
Effective Ground-Fault Current Path—An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current during ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems.
Ground—The earth.
Grounded (Grounding)—Connected (connecting) to ground or to a conductive body that extends the ground connection. (The neutral conductor is a grounded conductor).
Grounded Conductor—A system or circuit conductor that is intentionally grounded.
Grounding Conductor, Equipment (EGC)—The conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. (AKA, typically green wire or bare copper wire).
Main Bonding Jumper—The connection between the grounded circuit conductor and the equipment grounding conductor at the service.
Neutral Conductor—The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.
Separately Derived System—A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.
System Bonding Jumper—The system bonding jumper performs the same electrical function as the main bonding jumper in a grounded ac (i.e., alternating current) system by connecting the equipment grounding conductor(s) to the grounded circuit conductor either at the source of a separately derived system or at the first disconnecting means supplied by the source. The term ‘system bonding jumper’ is used to distinguish it from the main bonding jumper, which is installed in service equipment. A system bonding jumper is used at the derived system if the derived system contains a grounded conductor. Like the main bonding jumper at the service equipment, the system bonding jumper provides the necessary link between the equipment grounding conductors and the system grounded conductor in order to establish an effective path for ground-fault current to return to the source.
It is important to note that the effective ground-fault current path (typically the EGC) is intended to carry current only during ground-fault conditions. Steady-state current flow on the EGC violates the NEC and can result in safety hazards, fire risks, and the misoperation of electrical equipment.
Another important note is that the designs of grounded low-voltage electrical systems allow only a single N-G bond per separately derived system (wherein “low-voltage” refers to the range of about 50 volts ac (root-mean-square (rms) voltage) to about 1000 volts ac or about 120 volts dc to about 1500 volts dc, in accordance with the International Electrotechnical Commission's (IEC's) definition). To further illustrate this point, FIG. 1 generally exemplifies the basic layout of a properly bonded three-phase electrical system 1 having a main panelboard 10 that supplies a 480-volt rated line voltage via conduit 17 to the delta primary of a delta/wye stepdown transformer 12, the secondary of which supplies a three-phase, 4-wire 208Y/120V system via subpanel 14, which includes a conductive media (e.g., bus bars, conductors, etc.) for distributing the phases 13, neutral (grounded) conductor N, and grounding conductor G to loads (e.g., sensitive equipment) and possibly also to one or more further separately derived systems.
As shown, the 208Y/120V system has a single point of bonding between the grounding conductors G and the grounded conductors N at the enclosure of the step-down transformer 12, which is connected to the grounding system 19 (e.g., comprising a grounding electrode conductor and a grounding electrode). Grounding conductor G is not intentionally connected (nor, therefore, ground bus bar 21) to the enclosure of subpanel 14, which is grounded by a supplemental means (e.g., a separate grounding system), or by grounding system 19 via a raceway between the enclosures of subpanel 14 and transformer 12. Unless a new electrical system is derived (e.g., another transformer downstream from the subpanel), there shall be no other bonds between the grounded conductor (e.g., the neutral wire) and the grounding conductor (e.g., the green wire, green-yellow wire, bare copper wire). The reason this configuration is not allowed is that supplemental N-G bonds will provide a complete path resulting in steady-state current flow on the grounding conductor. As previously mentioned, ground currents can result in safety hazards and equipment reliability issues.
Measuring N-G voltages inherently requires a known impedance (Z) between the N-G so that the small current flow across that known impedance can be used to calculate the voltage via Ohm's Law. As a practical matter, this impedance (Z) is essentially a high-resistance connection between the grounded conductor and the grounding conductor. Assuming the impedance (Z) is high, the current flow will be small. Applying multiple metering devices or intelligent electronic devices (IEDs) on the same source, however, will result in additional current flow that is dependent on the number and location of the metering devices or IEDs and their respective impedances between the grounded and grounding conductors. Generally, however, N individual impedances on the same source will add in parallel to produce an equivalent impedance; as such, where each of N individual impedances have equal impedance, the equivalent impedance will be the individual impedance divided by N.
FIGS. 2a-2c below provide an example of the impact of multiple meters measuring the N-G voltage on an electrical system. In FIG. 2a, a single meter with a N-G voltage of 50 volts across 1 MΩ of impedance results in 0.05 mA of steady-state current between the neutral and ground. Two meters with the N-G voltage and impedance will result in twice the steady-state current (0.10 mA) flowing from the neutral to the ground as shown in FIG. 2b. Finally, FIG. 2c shows N meters with a N-G voltage of 50 volts across 1MΩ of impedance in each respective meter resulting in N×0.05 mA of current flowing from neutral to ground. It is unlikely that a typical installation would experience a potential of 50 volts between its neutral and ground; however, simply measuring the N-G potential will result in current flow. Thus, more installed meters (or other devices with an impedance between neutral and ground) with an impedance between the neutral and ground (whether the intention is to measure the N-G voltage or not) can result in elevated steady-state current flow on the ground path. Furthermore, metering devices or IEDs with the ability to measure N-G voltages are becoming more prevalent. FIG. 2D provides one example of the potential N-G current paths in a small metering system.
The 2011 NEC Codebook refers to this type of current as “objectionable current” due to the undesirable impact it can have on electrical systems as previously discussed. Possible concerns that can result from objectionable current flow on the ground path, for example, include (1) safety issues (as described in the NEC), and (2) signal and performance reference issues (as described in IEEE Std. 1100-2005). Both directly impact the ability of an energy consumer to safely and/or effectively meet their commercial objectives.